The present invention relates to an evaporation source material for vapor deposition which can be used for forming a magnesium oxide thin film through a vapor deposition process, in particular, an electron beam (EB) vapor deposition process, or an ion plating method utilizing the EB vapor deposition, and to a method of forming a transparent barrier film which is excellent in gas-barrier property especially against oxygen gas and moisture.
The vapor deposition technique of magnesium oxide and a thin film obtained from the vapor deposition of magnesium oxide are utilized in various fields such as precision industry, electric industry, package industry, etc.. For example, the magnesium oxide is deposited on a base film to form thereon a transparent barrier film. The barrier film is coated by a sealable film to thereby produce a composite packaging film.
In order to form a magnesium oxide thin film by vacuum vapor deposition process, magnesium metal in combination with oxygen as a reaction gas, magnesium oxide powder or compression-molded product of the magnesium oxide powder is conventionally used as a vapor deposition source material.
However, in the conventional method of using magnesium metal, since magnesium vapor evaporated upon heating reacts with oxygen gas to form a film of magnesium oxide, it is difficult to control composition of the resultant film. Further, since growth rate of the film is dependent upon and limited by the reaction rate between the magnesium vapor and oxygen gas, it is difficult to achieve high speed vapor deposition of magnesium oxide.
Meanwhile, most of the conventional pulverized magnesium oxide material is about 1.5 g/ml or less in bulk density, and 10 .mu.m or less in grain size. Accordingly, it is impossible to densely pack the source material in a crucible or hearth, so that gas tends to be kept within the source material filled in the crucible and the like. Evacuation rate of a chamber is rather slow due to outgas from the source material, thus badly affecting a vacuum degree within the evaporation chamber.
Further, since the packing density of the conventional magnesium oxide source material filled in the crucible and the like is low as mentioned above, and excavation rate of the source material by irradiation of electron bean is too fast, thereby hindering continuous evaporation operation for a long period of time. Moreover, when power of the electron beam is increased, splash or scattering of the source material is caused to occur, thereby rendering the deposition thereof to become unstable. The conventional pulverized source material is rather chemically active, larger in active surface, prone to absorb water and carbon, and vulnerable to denaturing, hence it gives rise to problem of preservation conditions such as short life time. When the source material thus absorbed with water and the like and thus denatured is subjected to the irradiation of electron beam, it will give rise to problems of volume shrinkage and splashing of the source material, thereby causing the vapor deposition to become highly unstable.
In order to avoid these problems, compression-molded product of the powdery source material has been conventionally employed as an evaporation source. This compression-molded product is generally produced by means of hot press. However, the compression-molded product produced in this manner has a bulk density of at most 2.5 g/ml. Some degree of improvements may be achieved with the employment of this compression-molded product. Namely, the bulk density thereof is fairly increased, and the excavation of hole in the source material can be suppressed as compared with the powdery source material. However, the compression-molded product is susceptible to volumetric shrinkage and crack when it is irradiated by electron beam, thereby causing release of gas confined in the compression-molded product, thus instabilizing the degree of vacuum in the evaporation chamber. Further, crushed fragments of the source material vary in size and shape so that exposed surfaces irradiated with electron beam are irregular, thereby instabilizing the evaporation rate. Further, tiny pieces resulting from the crush of the source material may become a cause of splashing. In case of a molded product shaped by using a binder, the volumetric shrinkage and release of outgas are more conspicuous thereby causing deposition of the source material to become more unstable. Further, the production of compression-molded product by using powder raw material gives rise to various problems such as an increase in manufacturing steps, and adjustment of the shape of the molded product to internal shape of the crucible or hearth, thereby extremely increasing the production cost. When the temperature of molding the raw material is raised in view of increasing the bulk density thereof, reduction reaction of magnesium oxide will be caused to occur. On the other hand, when the pressure of molding the raw material is increased, high pressure resistance or rigidity of a press mold may be required, thereby extremely increasing the manufacturing cost.
When the powdery source material or compression-molded source material as mentioned above is employed, the following various problems are imposed on the production of a transparent barrier film due to the defects as explained above in conjunction with the evaporation source material. Their problems associated to the transparent barrier film include formation of pinhole in the resultant deposited film, nonuniformity in thickness of the magnesium oxide layer due to the unstable evaporation, difficulty of performing a high speed deposition under a high feeding speed of a base film web due to limited evaporation rate under a limited degree of the power of electron beam, and incapability of performing a long term evaporation due to quick excavation of holes in the evaporation source.